Carbon dioxide is a colorless, odorless gas. It occurs in the atmospheres of many planets, including that of the earth. On the earth, all green plants must get carbon dioxide from the atmosphere to live and grow. Animals produce the gas when their bodies convert food into energy and living tissue. Animals release carbon dioxide into the atmosphere. Carbon dioxide is also created by the burning of any substance that contains carbon. Such substances include coal, gasoline, and wood. Fermentation and the decay of plants and animals also produce carbon dioxide (see Fermentation). Carbon dioxide makes up less than 1 percent of the earth's atmosphere.